1. Field of the Invention
The present invention relates to an apparatus for controlling a change of the internal pressure of a fuel tank caused by evaporation of a liquid fuel that is stored within the fuel tank. More particularly, it relates to an apparatus for controlling internal pressure in a fuel tank that prevents fuel components from escaping out of the fuel tank.
2. Description of the Related Art
Liquid fuel such as gasoline may be store in a fuel tank. When a gaseous layer space is formed in an upper portion within the fuel tank as am amount of the liquid fuel within the fuel tank is reduced, vapor generated (due to the evaporation of the liquid fuel such as evaporated fuel) may be filled in an upper space within the fuel tank. In addition to fuel components, air components are also contained in the vapor. Therefore, when a vehicle is exposed to heat over a long period of time and a temperature within the fuel tank is increased because of a high ambient temperature, the liquid fuel begins actively to evaporate. Consequently, an internal pressure within the fuel tank is increased caused by the expansion of vapor that may cause a damage of the fuel tank. In order to avoid the damage of the fuel tank, it is necessary to exhaust the vapor within the fuel tank to outside in accordance with the internal pressure within the fuel tank. However, it may cause a waste of fuel if the fuel components are escaped as the air components are exhausted. In order to prevent a waste of fuel, conventionally an apparatus for controlling a change of internal pressure within a fuel tank is known as disclosed in Japanese Laid Open Utility Model Publication No. 58-64854. According to such a conventional apparatus, vapor within a fuel tank may be exhausted via a canister containing a porous material, such as an activated carbon in order to control a change of internal pressure within a fuel tank. The fuel components may be captured by the porous materials as the vapor within the fuel tank passes through the canister so that only air components can be exhausted to the external atmosphere. Further, the canister communicates with an intake pipe of an engine and is constructed to supply fuel components into an engine. The fuel components captured by the porous material of the canister are desorbed (purged) due to a negative pressure in an intake pipe that is generated when the engine is started, and are then supplied into the engine.
In order to recover an ability of the canister to capture fuel components, it is necessary to sufficiently desorb captured fuel components. Generally, it is known that a required amount of desorbed air is 300 to 600 times more than a capacity of a canister. Therefore, an amount of desorption and time for desorption depend on a capacity of a canister. However, the recent hybridization of vehicles and the economy run technology have caused an increase of an engine stop time during driving. Also, negative pressure in an intake pipe has been reduced due to a loss reduction technology of a pump for an engine. Because of these factors, it is hardly to obtain a sufficient time for desorption using the configuration as disclosed in Japanese Laid-Open Utility Model Publication No. 58-64854.
Japanese Laid-Open Patent Publication No. 6-74118 discloses an apparatus having a fuel vapor separator mounted on an upper portion of a fuel tank. The fuel vapor separator is filled with porous materials, such as zeolite that permits the passage of air components but does not permit the passage of fuel components. The vapor within the fuel tank can be exhausted via a canister as well as the fuel vapor separator to the atmosphere. This construction can only exhaust air components out of the fuel tank in accordance with the internal pressure while the capacity of the canister is maintained at a low level. Because the capacity of the canister can be maintained at a low level, the time consumed for purging fuel components may be saved. When the internal pressure of the fuel tank is reduced to lower than the atmospheric pressure, external air is introduced into the fuel tank via the fuel vapor separator.
The fuel vapor separator according to the Japanese Laid-Open Patent Publication No. 6-74118 utilizes a porous material having a pores diameter of approximately 3.5 to 4.0 angstroms that permits the passage of air components but does not permit the passage of fuel components. However, diameters of pores of the porous materials are not uniform. Therefore, it is usual that some pores may have a size that permits the passage of the fuel components. Accordingly, the evaporated fuel control apparatus as disclosed in Japanese Laid-Open Patent Publication No. 6-74118, which exhausts vapor within the fuel tank to the atmosphere via the fuel vapor separator may permit the passage of the fuel components out of the fuel tank depending on the capability of the fuel vapor separator. Consequently, it cannot reliably avoid a loss of fuel.
Thus, in view of the above-described problems, the present invention has found that the loss of fuel components may be prevented if a change of internal pressure could be controlled by a porous material, which is not communicated with external air instead of selectively exhausting vapor within the fuel tank. It is therefore an object of the present invention to provide an apparatus for controlling internal pressure of a fuel tank that prevents fuel components from escaping out of the fuel tank without, being influenced by the engine condition, as the internal pressure within the fuel tank changes.